As the bullet flies over the first magnet, it disrupts the magnetic field, which generates a spike in voltage. This spike triggers a clock. Once the clock starts, the first sensor takes another time stamp as the center of the bullet passes over it (see graph). The sensor detects this midpoint as the initial voltage spike drops to nearly zero, which is the “magnetic center” of the projectile. The second sensor takes two similar time stamps, the last of which stops the clock. Because the distance between the two sensors is precisely calibrated—they are 5 inches apart, +/-.0005 of an inch, according to the company— the time of flight between the sensors gives the speed of the bullet.